Which of these numbers is prime? ${21,\ 31,\ 54,\ 55,\ 81}$
Explanation: A prime number has exactly two factors: 1 and itself. The factors of 21 are 1, 3, 7, and 21. The factors of 31 are 1 and 31. The factors of 54 are 1, 2, 3, 6, 9, 18, 27, and 54. The factors of 55 are 1, 5, 11, and 55. The factors of 81 are 1, 3, 9, 27, and 81. Thus, 31 is a prime number.